R7986ATR datasheet(19/37 Pages) STMICROELECTRONICS

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DocID026997 Rev 1

19/37

R7986A

Application information

6.4

Compensation network

The compensation network must assure stability and good dynamic performance. The loop

of the R7986A is based on the voltage mode control. The error amplifier is a voltage

operational amplifier with high bandwidth. So by selecting the compensation network, the

EA is considered as ideal, that is, its bandwidth is much larger than the system one.

The transfer function of the PWM modulator, from the error amplifier output (COMP pin) to

the OUT pin (see Figure 10) results:

Equation 16

where VS is the sawtooth amplitude. As seen in Section 5.1 on page 9, the voltage feed-

forward generates a sawtooth amplitude directly proportional to the input voltage, that is:

Equation 17

In this way the PWM modulator gain results constant and equal to:

Equation 18

The synchronization of the device with an external clock provided trough the SYNCH pin

can modify the PWM modulator gain (see Section 5.1 on page 9 to understand how this gain

changes and how to keep it constant in spite of the external synchronization).

Figure 9. The error amplifier, the PWM modulation and the LC output filter

GPW0

VIN

---------

KVIN



GPW0

VIN

---------

----

COMP

REF

E/A

PWM

OUT

ESR

PW0

COMP

REF

E/A

PWM

OUT

ESR

PW0

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